Atherosclerosis is the thickening and loss of elasticity in the inner walls of arteries, accompanied by the formation of small fatty modules on the artery walls and degeneration of the affected area. Atherosclerosis presented in the form of coronary heart disease and cerebrovascular diseases are major causes of morbidity and mortality in many industrial countries. Elevated plasma levels of low density lipoprotein-cholesterol complex (LDL-C) correlate with an increased risk for the development of atherosclerosis.
Patients at high risk for atherosclerosis are encouraged to make dietary changes in an attempt to control LDL-C levels. However, patient compliance is not always high and there is a large patient population which cannot control LDL-C levels merely through dietary modifications.
Drug therapy is also commonly used to try to lower LDL-C levels. While drug therapy is effective for many patients, there are still a large number of patients who are resistant to drug therapy or who suffer too many side effects to warrant its use.
In addition to dietary changes and drug therapy, attempts have been made to remove LDL-C directly from the plasma of patients through extracorporeal methods. These methods include plasma exchange, filtration based on molecular size, immunoadsorption, heparin precipitation and dextran sulfate adsorption. While these methods effectively remove LDL-C from plasma, they also remove varying quantities of desirable plasma components. The plasma exchange method removes all plasma and replaces the volume with plasma or albumin replacement solutions. All valuable plasma components, such as high density lipoprotein (HDL), and proteins are removed in addition to the LDL-C. The other methods, while better than plasma exchange, have varying degrees of specificity for only LDL-C. With filtration based on molecular size, there is considerable loss of proteins with molecular weights greater than 250-400 kD. Immunoabsorption is specific for LDL-C only, but its efficiency for removal of LDL-C is not as great as other methods. Heparin precipitation and dextran sulfate adsorption remove LDL-C, but a loss of 20-40% of HDL is generally expected; also the adsorbing capacities are fairly low. Since HDL plays an important role in reducing a patient's risk for atherosclerosis, a method which eliminates or minimizes the loss of HDL is highly desirable.
Previous filtration methods have also utilized carriers, such as agarose beads, which lack mechanical strength, and as a result are difficult to handle and operate. When fluid is passed through these carriers, there is a high probability of blockage. Additionally, these carriers may be destroyed by sterilization techniques. These carriers might also leach materials into the patient fluid.
Polyacrylate has been tested as a sorbent for lipoproteins from human plasma (Thies et al., Artificial Organs (1988) 12(4):320-324). Negligible loss of HDL and plasma proteins was shown with this absorbent. Polyacrylate has been attached to cellulosic beads through amide linkages. While the preparation was useful, it was not optimal for the treatment of whole blood. As mentioned previously, cellulosic beads do not have good mechanical strength, block easily, and are not easily sterilized.
Kuroda et al. (EP 0143369) describe a porous adsorbent for absorbing low density lipoproteins having a silanol group and a synthetic polyanion linked with the surface. To prevent clogging, the porosity of the adsorbent must be distributed over a broad diameter range. By contrast, the microporous membrane of the present invention has uniform pore diameters. Murakami (Japanese P.A. 01-229878) describes porous polyester fibers coated with methacrylic acid which are useful to remove bilirubin or LDL from body fluids. Sterilization of polyester fibers can be problematic. Kuroda et al. (Japanese P.A. 63-232845) describe an absorbent material having on its surface a synthetic linear polymer which has both a carboxyl group and sulfate or sulfonate groups.
To date, the majority of extracorporeal methods for the removal of LDL-C have involved two separate steps. First, the blood must be separated into cellular components and plasma components. This is usually done through centrifugation or filtration. Second, the plasma is treated to remove LDL-C. Finally, the treated plasma and cellular components are returned to the patient. The procedures are both time consuming and require a great deal of handling of blood products, which leads to increased potential for infections. Methods involving a closed system which are relatively rapid, efficient and require limited handling of blood are highly desirable.